The invention relates to the checking of the calibration of a display system. A display system is, for example, a monitor which may or may not be associated with its graphics card or a printer producing a paper proof observed in the light of an office, or a laser reprographics device producing a film observed in a negative viewer, or optionally a combination of several of these display means.
A display means of a display system, for example connected to a medical imaging apparatus, receives as input a digital image, each point of which is assigned a brightness value (generally between 0 and 255) which will be referred to below by the expression "digital grey level". Each pixel of the image reproduced by the display means then has a brightness level whose value depends on the digital grey level of the corresponding point in the input digital image. The transfer function of a display means is the correspondence law between the input digital grey levels and the output brightness levels.
A display means is said to be calibrated when its transfer function coincides with a predefined transfer function, called below the "target transfer function".
Among known calibration methods, mention may be made of those which use generic digital images, i.e. digital images that are independent of the target transfer function and of the observation conditions, in order to test the actual transfer function of a display means. This method of the prior art therefore uses optical density measurements or brightness levels of the various grey levels of these digital test images reproduced by the display means.
The invention aims to provide a radically different solution to the problems of calibrating display systems.